Do nothing other than not answer. If you set up groups or anything which makes a connection with their phone or computer, they will keep calling. If you do not connect, they will drop your number after a few tries.
I just want to add on to the comment first made by Sandra15, and then followed up on by Spammed Out about how best to notify yourself of unwanted "spam" calls. To add to that some cell phones allow you to send calls right to voice mail. So after I get a call the first time from a number I know, or find out is worthless spam I add it to a contact called Send To Voicemail and have that contact set up to just go to voice mail. That way I am never disturbed in the least since the phone never even rings, so I don't know the call even came in, unless I look at the call history. Just an idea to only have to waste your time once with these types of worthless calls.
I didn't answer because I didn't recognize the number. Then I called back, using *67 to mask my number. I heard a recording saying, "The number you are trying reach does not have a voice mail box set up. Hang up and try again later." I called two more times, without using the *67 function, and got the same recording.